thanks for the reply, but i dont really get it, by perpendicular does that mean a line from v to center of the square? And what does "prove it" mean, do i use Pythagoras or another equation?

the base of the pyramid is a square... V is a point in space. now we are dropping a perpendicular to the base from V. the foot of perpendicular HAPPENS to be the centre of the square base and this is what has to be proved when i said "prove it".